The invention relates generally to position sensing of hydraulic and pneumatic actuators. More particularly, it relates to sensing using laser light sources and detectors and determining the position of the actuator using time-of-flight algorithms.
Position sensing for hydraulic or pneumatic actuators typically uses an external position sensor, such as a rotary rheostat or potentiometer. Alternatively, linear rheostats or variable differential transformers are employed. These systems suffer from poor accuracy, extensive wear, and fragility in many applications, especially demanding applications such as their use on work and agricultural vehicles.
These sensors are quite susceptible to damage, and suffer from being damaged during vehicle operation, or from the extremes in temperature that work and agricultural vehicles face.
In an effort to solve these problems, new methods of measuring the position of a hydraulic or pneumatic actuator have been devised that use microwaves. These waves are transmitted from one end of the cylinder, reflect off the piston, and return to a detector. By measuring the time-of-flight of these waves, the location of the piston can be determined. Such an example is shown in U.S. Pat. No. 6,005,395, which is incorporated herein by reference for all that it teaches.
The microwave transmitter suffers from high cost and difficulties in determining which of the many reflections in the cylinder is the proper one to measure.
In an alternative system, the pulse generating and timing circuits of U.S. Pat. No. 6,005,395 are used, but are coupled to a laser light source and respond to a reflection of that beam against a laser light detector, such as that shown in co-pending U.S. patent application Ser. No. 09/750,866.
This arrangement also has drawbacks. When the piston moves toward or away from the source and detector, the reflected light follows multiple paths that, like the microwave transmitter and receiver pair, make the reflected pulses difficult to interpret. It is difficult to extract a good pulse indicative the precise time-of-flight of the laser beam.
An improvement on this system is provided in our co-pending application entitled xe2x80x9cMULTI-FIBER CYLINDER POSITION SENSOR USING TIME-OF-FLIGHT TECHNIQUExe2x80x9d, docket number 13936 and filed contemporaneously herewith. In that application, a single optical fiber transmits laser-light pulses from outside a hydraulic or pneumatic cylinder to inside the cylinder. The fiber is preferably located along a central longitudinal axis of the cylinder. The light pulses from the transmitting fiber travel down the cylinder substantially parallel to the longitudinal axis of the cylinder and reflect off the face of the piston in the cylinder. The light is reflected straight back toward the transmitting fiber. The path it follows in returning to the transmitting fiber at the end of the cylinder is substantially the same path as the path it traveled when going from the fiber to the piston. In short, the laser beam is preferably normal to the piston where it is reflected in order to provide these parallel in and out paths. When the laser light pulses return to the region of the transmitting fiber, they fall on the free ends of several optical fibers disposed around the central transmitting fiber. All of these fibers receive the light pulses at substantially the same time and conduct the light pulse from inside the cylinder to outside the cylinder. The receiving fibers are closely spaced in a circular arrangement equidistant from the central fiber. Since the light pulse from the central fiber follows the same path back after reflecting from the piston, each of the fibers receives approximately the same amount of light energy, and receives it at almost exactly the same time.
The distal ends of the receiving fibers are coupled together such that each portion of the reflected light pulse that each individual fiber of the receiving fiber carries are merged to form a much stronger light pulse. The lengths of the receiving optical fibers are chosen such that the portions of the reflected light pulse that each one carries is merged into a single pulse at exactly the same time. This sharply increases the magnitude of the resulting pulse and provides an extremely fast and sharp rise time. In this manner, a reflected light pulse can be xe2x80x9creassembledxe2x80x9d with a very sharp leading edge that permits precise time-of-flight measurements.
The system described in the foregoing patent application, however, discloses a separate laser diode and separate photodiode for use with a single cylinder. In addition, there is complex and expensive circuitry to expand the light pulse and compare the phases of the transmit and receive pulses to determine the time-of-flight in a cylinder, and thereby the position of the piston within the cylinder.
Duplicating this structure in a vehicle that has several hydraulic or pneumatic cylinders would be prohibitively expensive. Multiplying the arrangement of the 13936 application would require as many laser diodes, photodiodes, amplifier circuits, pulse expansion circuits and phase comparators as there are individual cylinders. What is needed, therefore, is a system that can measure the position of several hydraulic cylinders, yet does not require duplicate sets of circuitry for each of those cylinders. It is an object of this invention to provide such a system.
In accordance with a first embodiment of the invention, a multiple cylinder position sensing system is provided that includes a first cylinder including a first source light guide having a first end and a distal second end and extending from inside the cylinder to outside the cylinder and adapted to transmit at least a first beam of laser light at a first frequency from outside the cylinder to inside the cylinder, and at least one first reflected light guide having a first end and a distal second end and extending from inside the cylinder to outside the cylinder and configured to receive light from the first beam of laser light that is reflected off the inside of the first cylinder, and a second cylinder including a second source light guide having a first end and a distal second end and extending from inside the cylinder to outside the cylinder and adapted to transmit at least a second beam of laser light at a first frequency from outside the cylinder to inside the cylinder, and at least one second reflected light guide having a first end and a second end and extending from inside the cylinder to outside the cylinder and configured to receive light from the second beam of laser light that is reflected off the inside of the second cylinder.
The system may include a laser light source that is optically coupled to the distal ends of both the first and second source light guides, and configured to generate a source beam of laser light, wherein the source beam is divided into the first and second beams of laser light. The system may include a first photodiode configured to receive and electrically respond to light from the first beam of laser light that is reflected off the inside of the first cylinder from the first reflected light guide. The system may also include a laser light source driver circuit coupled to the laser light source and configured to energize the laser light source upon receipt of a trigger pulse, and a timing circuit coupled to the laser light source driver configured to generate the trigger pulse and apply the trigger pulse to the laser light source driver circuit. The laser light source may be a laser diode. The system may include first and second photodiode amplifiers that are coupled to the first and second photodiodes, respectively.
Each of the first and second photodiode amplifiers may be configured to generate an output signal.
The system may also include a pulse expansion circuit, to which the first and second photodiode output signals are coupled.
The second ends of the plurality of second light guides may be optically coupled to a single light detector. The light detector may have an electrical output that is produced by light carried by at least two of the plurality of second light guides.
In accordance with a second embodiment of the invention, a method for determining the time-of-flight of laser light pulses in a plurality of hydraulic or pneumatic cylinders is provided, including the steps of generating a timing pulse in a timing circuit, conducting the timing pulse to a laser light source and responsively generating laser light pulse from the source, conducting a first portion of the pulse through a first optical fiber to a first cylinder, conducting the first portion into the first cylinder, reflecting the first portion off a first reflective surface coupled to a first piston in the first cylinder, receiving the first portion at a first photo diode and responsively generating a first electrical signal, conducting a second portion of the pulse through a second optical fiber to a second cylinder, conducting the second portion into the second cylinder, reflecting a second portion off a second reflective surface coupled to a second piston in the second cylinder, receiving the second portion at a second photo diode and suppressing the generation of the second electrical signal, providing the first electrical signal and the timing pulse to a comparator circuit and responsibly generating a first output signal indicative of a first time difference between the arrival of the timing pulse and the arrival of the first electrical signal at the comparator circuit.
The method may also include the steps of generating a second timing pulse in the timing circuit, conducting the second pulse to the laser light source and responsibly generating a second laser light pulse from the source, conducting a first portion of the second laser light pulse through the first optical fiber to the first cylinder, conducting the first portion of the second laser light pulse into the first cylinder, reflecting the first portion of the second laser light pulse off the first reflective surface, receiving the first portion of the second laser light pulse at the first photo diode and suppressing the generation of a third electrical signal indicative of the time of arrival of the first portion of the second laser light pulse at the first photo diode, conducting a second portion of the second laser light pulse through the second optical fiber to the second cylinder, conducting the second portion of the second laser light pulse into the second cylinder, reflecting the second portion of the second laser light pulse off the second reflective surface, receiving the second portion of the second laser light pulse at a second photo diode and responsibly generating a fourth electrical signal indicative of the time of arrival of the second portion of the second laser light pulse at the second photo diode, providing the fourth electrical signal in the second timing pulse to the comparator circuit and responsibly generating a second output signal indicative of a second time difference between the arrival of the timing pulse and the second electrical signal at the comparator circuit.
The step of conducting the first timing pulse to the laser light source and responsively generating a second laser light pulse from the source may include the steps of optically coupling the laser light source to distal ends of the first and second optical fibers, and dividing the first laser light pulse into the first and second portions. The method may also include the steps of providing a laser light source driver circuit, coupling the laser light source to the driver circuit, applying the first and second timing pulses to the laser light source driver circuit, and energizing the laser light source responsive to the application of the first and second timing pulses to the driver circuit. The method may include the steps of providing a first photo diode amplifier and coupling the first photo diode amplifier to the first photo diode, providing a second photo diode amplifier and coupling the second photo diode amplifier to the second photo diode, generating a first gate signal in the timing circuit, applying the first gate signal to the first photo diode amplifier to permit the transmission of first electrical signal, generating a second gate signal in the timing circuit, and applying the second gate signal to the second photo diode amplifier to suppress the transmission of the second electrical signal. The method may include the step of configuring the first and second photo diode amplifiers to generate first and second amplifier output signals, respectively. The method may include the step of coupling the first and second photo diode amplifier output signals and transmitting the coupled output signals to a pulse expansion circuit. The method may include the step of transmitting the first and second output signals to a pulse expansion circuit. The method may include the steps of generating an expanded pulse output signal in the pulse expansion circuit, and outputting the expanded pulse output signal from the pulse expansion circuit. The method may include the steps of providing a pulse comparator circuit, and inputting the expanded pulse output signal and the timing pulse into the pulse comparator circuit, and generating a time delay output signal in the pulse comparator circuit indicative of a time delay between the timing pulse and the expanded pulse output signal.
In accordance with a third embodiment of the invention, a method of determining the time-of-flight of laser light in a plurality of hydraulic or pneumatic cylinders includes the steps of transmitting a laser light pulse from a laser diode, dividing the laser light pulse into at least first and second sub-pulses, injecting the first and second sub-pulses into first and second cylinders, respectively, reflecting the first and second sub-pulses off first and second pistons in the first and second cylinders, respectively, transmitting the first and second reflected sub-pulses at two first and second photo diodes, respectively, generating first and second electrical signals in the first and second photo diodes that are indicative of the first and second times of arrival of the first and second sub-pulses at the first and second photo diodes, respectively, selectively coupling the first and second electrical signals in a first mode of operation to a pulse expansion circuit and a phase comparator circuit to generate a first time-of-flight signal on an output line of the phase comparator circuit that is indicative of the time-of-flight of the first sub-pulse and not of the second sub-pulse, repeating the foregoing steps with a second pulse of laser light, but in a second mode of operation wherein the phase comparator circuit generates a second time-of-flight signal on the output line that is indicative of the time-of-flight of the second sub-pulse and not of the first sub-pulse of the second pulse of laser light.